The basic structures of a light-emitting diode (LED) include a p-type semiconductor layer, an active layer (light-emitting layer) and an n-type semiconductor layer. When the LED is under forward bias, the hole in the p region moves towards the n region and the electron in the n region moves towards the p region. The electron and the hole are recombined in the active layer. In theory, the energy generated from the electron and hole recombination is mostly released by light (radiative transition). It can be seen that the LED can directly transfer the electricity into light, and has such advantages as high energy conversion efficiency and small volume. In addition, the LED is free of poisonous and harmful substances and meets environmental and sustainable development requirement. The LED has broad market development prospect thanks to low energy consumption, environmental protection, small volume and long service life.